The invention relates to a method for producing permanent integral connections of oxide-dispersed (ODS) metallic materials, in particular for producing integral connections of components of oxide-dispersion-strengthened noble-metal alloys, specifically with successive heating steps and mechanically shaping the bond at the connection.
It is known in the production of special types of glass to use structural elements encased in noble metal, for example stirrers, channels, feeder heads, for homogenizing or transporting the glass melt. The noble-metal alloys used are usually platinum base alloys with alloying additions of rhodium, iridium or gold. If very high component strengths are required on account of mechanical 1 and/or thermal loads imposed by the glass melt, oxide-dispersion-strengthened platinum base alloys are increasingly used, since they are characterized by a greater thermal, mechanical and chemical load-bearing capacity than standard alloys. Oxide-dispersed alloys, referred to hereafter as ODS alloys, are distinguished by a very homogeneous microstructure. ODS noble-metal alloys based on PtRh10, PtAu5 or pure Pt, which are used for producing components in the glass industry, additionally have a much lower tendency for coarse grains to form at high temperatures, allowing the mechanical properties to be positively influenced. Apart from the choice of suitable material, however, the production, in particular shaping, of the structural elements encased in noble metal also plays an important part in determining the strength. They are generally produced from semifinished products, i.e. metal sheets or sheet-metal elements are joined together to give the required geometry. This connection is generally produced by fusion welding of the individual semifinished products. In this case, the joints of the components to be connected to one another and, if appropriate, additional material of the same type are transformed into the molten state by heat being supplied and they are fused together. The heat of fusion may in this case be generated by an electric arc or an ignited gas-oxygen mixture. If, however, structural elements joined in such a way are exposed to very high temperatures, for example above 1200° C., the welded seam often forms the weak point of the overall material bond. Inhomogeneities in the welded seam and changes in the microstructure in the heat-affected zone have been determined as the cause. Particularly longitudinal welded seams in cylindrical components, such as pipes for example, are particularly at risk because of the stresses acting that are approximately twice as high as in the case of circumferential welded seams, with the result that the seams often fail and tear apart. When known welding methods are used, such as for example tungsten-inert gas welding (TIG welding), plasma welding, laser or autogenous welding, melting of the alloy is brought about. While only very small losses in strength in the welded seam are to be observed during the melting of classic substitutional solid solution alloys as a result of recrystallization during use above 1200° C., the melting when oxide-dispersion-strengthened alloys are welded leads to coagulation and floating of a large part of the disperoids, typically of ZrO2 and/or Y2O3, in the melt. A coarse-grained solidification structure is produced in the welded seam. The strengthening effect of the dispersoids in the welded seam is consequently negated. The load-bearing capacity and service life of a structural element joined together in such a way then falls to the level of structural elements joined from standard alloys. Measures to avoid this disadvantage are already known from the publications JP 5212577 A and EP 0320877 B1. In the case of the methods disclosed therein, a fusion-welded seam on ODS sheets is subsequently covered with a Pt-ODS sheet and pressed into the seam by hammering at high temperatures. This measure brings about an increase in the fineness of the grain size distribution on the surface of the welded seam through the sheet and consequently a reduction in the probability of crack formation on the surface. A further possible way of compensating for the decrease in strength in the welded seam has been seen in forming the connection by means of flanged seams. However, these require undesired thickenings of the material in the joining region, which have the consequence when these components are heated in the direct current flow—for example for the purpose of heating the glass melts passed through structural elements joined in such a way—of producing differences in temperature at the seam, which in glass production may in turn lead to considerable glass defects. Furthermore, satisfactory beading of these thickened welded seams is possible only to a restricted extent. Alternatively, therefore, recourse is made to integral connections formed by means of hammer-welded seams. However, connections of this type are subject to very great variations in quality. To eliminate these variations, an extremely great expenditure is required for the preparation of the welded seam and very exact control of the process parameters during the welding. In the case of this method, uniform heating of the materials to be joined, in particular metal sheets, during hammering proves to be difficult. When doing so, it is often scarcely possible to heat the lower sheet in the welding position adequately with the torch to achieve a good adhesive effect during the hammering. The method is consequently very laborious, does not necessarily lead to an optimum result and is very cost-intensive. Furthermore, there is a fundamental problem when fabricating hammer-welded seams, in that there is a low adhesive tendency of the material during the hammering in the case of alloys with a rhodium content >4% by weight and in general in the case of ODS alloys. The oxides already contained in the ODS material or the oxides forming during the hammering, mainly rhodium oxide, significantly reduce the adhesive bonding of the two components, in particular metal sheets. The poor adhesive bonding has the effect of increasing the production expenditure considerably, but also at the same time of increasing the risk of no adequate bond being achieved any longer in certain regions of the joining region in the seam.
As a further possible way of producing welded connections of oxide-dispersion-strengthened alloys with high strength, welding with alloying additions containing zirconium and/or yttrium was considered. These alloying constituents should be separated during use at temperatures above 1200° C. by internal oxidation of the dispersoids in the material and consequently a strengthening effect achieved in the welded seam. In practice, however, it has been found that this method produces only inadequate results, since, during the separation of the dispersoids, the increase in the grain size also occurs at the same time. Consequently, a coarse-grained microstructure in which only a few dispersoids are separated, and the mechanical properties of which are therefore inadequate, often forms very quickly. Such a method for producing pipes from ODS noble metals is described in DE 197 14 365 A1. In the case of this method, as well as the heat treatment, additional rolling is required, whereby the working becomes very protracted and laborious.